The present invention relates generally to driver circuits for output switches, and more specifically to driver circuits with better Electro-Magnetic Interference (EMI) and propagation delay control.
An output switch may be used to provide a switching output voltage in response to a control signal from its driver circuit. Usually, a short propagation delay, the delay from the time of a control signal change to the time of a corresponding output voltage change, is preferred, since it is good for the stability of a circuit using the output voltage. At the same time, a slow transition on the output voltage may generate less EMI and thus is preferable. Prior art driver circuits for output switches have either a short propagation delay or a slow transition on the output voltage, but not both.
FIG. 1 illustrates a prior art driver circuit for an N-type output switch mn3. The driver is inverter based and has a pair of complementary field-effect transistors (FETs): a P-type FET mp0 and an N-type FET mn0. Their gates are coupled to an input switching voltage Vin, and their drains are coupled together. A power supply VDD is applied to the source of mp0, and the source of mn0 is grounded. The voltage at the drains of mp0 and mn0 is used to drive an output switch mn3, being applied to the gate of mn3 as Vg. The source of the output switch mn3 is grounded and an output voltage Vout is obtained from the drain of mn3. The threshold voltage of mn3 is VTH, and VTH<VDD.
FIG. 2 illustrates waveforms of Vin, Vg and Vout in the driver circuit of FIG. 1 during a process of turning off the output switch mn3. When the input voltage Vin is low, mp0 is conductive, mn0 is not conductive and Vg≈VDD. Consequently, mn3 is conductive, and Vout is low. When the input voltage Vin turns high, mp0 will stop being conductive when Vin reaches its threshold voltage, and mn0 will become conductive when Vin reaches its threshold voltage. When mn0 is conductive, it may pull down Vg. When Vg drops below VTH, the threshold voltage of mn3, the conductivity of mn3 reduces and Vout starts the transition from low to high.
A propagation delay is measured from the Vin change to the point at which the transition of the output voltage Vout starts. The propagation delay may be controlled by the strength of the driver, or mn0 more specifically, because the stronger the driving strength of mn0, the faster the Vg may be pulled down, and the shorter the propagation delay. FIG. 2 illustrates waveforms of Vin, Vg, and Vout when a mn0 with a strong driving strength is used, and FIG. 3 illustrates waveforms of these signals when a mn0 with a weak driving strength is used. As shown, when a mn0 with a strong driving strength is used, the propagation delay is short; but the transition on Vout is fast, resulting in more EMI. On the contrary, when a mn0 with a weak driving strength is used, the transition on Vout is slower, which is good for EMI performance, but the propagation delay is very long.
Adjusting mn0 current sink capability may change the output transition change rate, but it may affect the propagation delay as well.
Therefore, it would be desirable to provide a driver for an output switch, which has a short propagation delay and a slow transition on the output voltage of the output switch.